Archive for the ‘Oakland boulders’ Category

Rocks of Lakeside Park

17 February 2020

Lakeside Park has undergone a lot of changes since Edson Adams put Oakland’s first golf course there. For one thing — and the thing behind this post — over the year the city has brought in rocks to a place that originally had none at all. Some of them are boulders that hold plaques: I won’t be talking about those. This is about the other ones, the working rocks who have the basic job of standing in your way, like the guard rocks down at Middle Harbor Park.

I take a walk around the lake every week, but this last week I took a few extra ones to visit all the working boulders. I think there are three generations of them. Here’s a selection.

The main road through Lakeside Park appears to have the first generation. My working theory is that the city parks department tapped a stash of rocks that were acquired on its own properties, principally Joaquin Miller and Leona Heights Parks. That accounts for the following mix of rock types. The majority belong to the Leona volcanics, probably sourced from Leona Heights Park. They present many different textures with an underlying lithology of light-colored, strongly altered volcaniclastic material that takes on an orange iron-oxide glaze with exposure. These five specimens illustrate the range of this rock unit.


The other boulders include nondescript ones I can’t confidently identify. Behind the rear lawn-bowling field is this laid-back hunk of what sure looks like Sierran granite.

But there’s a specimen of serpentinite, worth a close look, next to the Nature Center.

And right in front is the lake’s special star: this wild, glittering piece of blueschist.

Another generation of boulders sits along the path in front of Children’s Fairyland. It too consists of local stones: besides the Leona volcanics it includes proper sandstone belonging to, if I’m not mistaken, the Oakland Conglomerate in Joaquin Miller Park.

Near the entrance is a splendid serpentinite boulder.

And best of all are some good specimens of the ocher-bearing material from the Leona volcanics that the Ohlone tribes once prized.

The third generation of stones is of recent vintage, installed during the park’s bond-funded upgrade. Their main hangout is on the shore east of the boathouse by the parking lot.

Another grouping is in the brand-new Snow Park extension at the foot of 20th Street.

When these went in I thought they were sandstone (and said so here), but upon closer inspection I conclude that they’re some sort of welded tuff, not from anywhere in the Bay area, probably some place across the Central Valley or the desert beyond. That’s OK — Oakland welcomes immigrants. The material is fairly featureless, but these rare clasts look like bits of country rock that got torn off and taken up during the eruptive cataclysm that made this stone.

The lake shore also has plenty of cut and dressed stone, in the form of benches and curbs and capstones. They’re all commercial quarry granite, hardworking stuff that will last forever, but without the personality of real live boulders.

A greenstone boulder in Lakeside Park

25 July 2016

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Lakeside Park holds a scattering of boulders and plaques. The plaques are always interesting, and sometimes so are the boulders. This one sits at the west side of Bandstand Cove by a grove of redwood and oak trees. I can tell at a glance — the greenish color, even texture and lack of sedimentary fabric — that this rock consists of metamorphosed lava, informally called greenstone. There’s a lot of it in the Coast Range. There’s also some in the Sierra foothills, and I suspect that this was quarried over there.

One side of the boulder displays a nice slickenside, a sign that the rock was cracked and wrung underground.

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Emily Brodsky down at UC Santa Cruz studies these fault surfaces and has been finding deep clues in them (see the latest paper from her team).

Elsewhere the boulder shows stretch marks — little extensional fractures filled with quartz. Like a run in a stocking, these are evidence of the stresses that affected this body of material once upon a time. Since the boulder has been ripped out of its original setting, these scrape marks and stretch marks have lost their geological meaning, but they’re still pretty.

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Oh yeah, the boulder has a message on it. The plaque announces that the three fountains in Lake Merritt were installed or renovated by Madeleine and Andrew Wong as a gift to the people of Oakland.

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And not least among its functions, the boulder punctuates the most peaceful view on the whole lake, whether the fountain is running or not.

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Lake Merritt needs a lot of human management to stay clean and pleasant, and the fountains are a key part of that.

Baldwin Street boulder

6 October 2013

Out in East Oakland at the corner of 85th Avenue and Baldwin Street is this fine, underappreciated boulder.

85th-ave-chert

To all appearances, it’s good old Oakland chert, hard at work. I assume it was put here to keep vehicles from cutting across the corner, or perhaps to keep a runaway vehicle out of the building behind it. Who knows? I was just glad it was there to break the monotony. It’s free of graffiti, too.

The other end of Baldwin Street is east of the Coliseum, where it serves as a back entrance for staff and athletes, at the edge of Arroyo Viejo. That’s the creek you cross when you’re walking from the BART station to the game. So between stone and water, Baldwin Street pays more homage to geology than most of its peers.

Lakeside Park terrace

1 September 2013

Lakeside Park is one of the most parklike parks I know. Perhaps I feel this way because I imprinted on it at kindergarten age.

Click for larger photo

Youthful feelings aside, I think that geology makes the park this way: it’s set on the late Pleistocene marine terrace, planed and beveled by the sea waves during an interglacial highstand approximately 125,000 years ago. The planar setting, studded with trees to the limits of vision, suggests a vision of infinity, or at least limitlessness.

This spot is in front of Children’s Fairyland, where young children then and now can experience mind-blowing things everywhere they look. Show them these rocks, too: they’re ocher-stone and chert from our own hills.